Solid polymer type fuel cells have been developed as clean power sources using hydrogen as fuel and as drive sources for electric vehicle and further stationary power sources for both generating power and supplying heat. Further, solid polymer type fuel cells feature a higher energy density compared with lithium ion batteries and other secondary cells. They are being developed as power sources for portable computers or mobile communication devices where high energy density is demanded.
A typical unit cell of a solid polymer type fuel cell is basically comprised of an anode (fuel electrode), a cathode (air electrode), and a proton conductive solid polymer electrolytic membrane arranged between the two electrodes. The anode and cathode are usually thin film electrodes comprised of a catalyst of platinum or another precious metal which is carried on a carrier carbon material, a fluororesin powder or other pore former, a solid polymer electrolyte, etc.
A solid polymer type fuel cell, as explained above, is a high energy density power source, but further improvement is being sought in the output per unit electrode area. For this reason, one of the most effective means for solution is to improve the catalyst activity of the electrochemical reactions occurring at the electrode catalysts forming the anode and cathode. In an anode using hydrogen as fuel, the catalytic activity of an electrochemical reaction where hydrogen molecules are oxidized to hydrogen cations (protons) is improved. On the other hand, in a cathode, the catalytic activity of an electrochemical reaction where protons from the solid polymer electrolyte react with oxygen whereby the oxygen is reduced to water is improved. For the electrode catalysts of the anode and cathode of such a solid polymer type fuel cell, platinum or another precious metal can be used. However, a precious metal is expensive, so to speed-commercialization and popularization of solid polymer type fuel cells, the amount of use per electrode unit area has to be reduced. For this reason, the catalyst activity has to be further improved.
Furthermore, when used as a fuel cell, it is known that starting and stopping or high load operation causes the catalyst ingredient platinum or other metal to be eluted or the carbon materials used for the carriers etc. to be corroded. For this reason, art for inhibiting elution of the platinum or other metal or corrosion of carbon is also extremely important.
As a measure for inhibiting corrosion of the carbon material which is used as a catalyst carrier, up until now, the following art has been disclosed. For example, PLT 1 discloses to use as the catalyst carrier a carbon material which is heat treated etc. to adjust the relative intensity ratio (ID/IG) of the peak intensity (ID) in the range of 1300 to 1400 cm−1 called the D-band and the peak intensity (IG) in the range of 1500 to 1600 cm−1 called the G-band, obtained from the Raman spectra, to 0.9 to 1.2.
PLT 2 discloses an electrode structure of a solid polymer type fuel which is high in power generation performance, high in potential durability, and excellent in durability against insufficient fuel by making the specific surface area of the carbon material which is used as the catalyst carrier 800 m2/g to 900 m2/g.
The basic structure of the solid polymer type fuel cell according to the present invention is that of a proton conductive electrolytic membrane sandwiched between an anode and cathode formed by catalyst layers which are in turn sandwiched at the outside by gas diffusion layers and at the further outsides by separators. These form a unit cell of a fuel cell. Usually, such unit cells of fuel cells are used stacked in accordance with the required output.
To take out current from this basic structure of fuel cell, oxygen or air or another oxidizing gas is supplied to the cathode side and hydrogen or another reducing gas is supplied to the anode side from the gas channels of the separators arranged at the two electrodes of the anode and cathode through the gas diffusion layers to the catalyst layers. For example, when utilizing hydrogen gas and oxygen gas, the energy difference (potential difference) of the chemical reaction of H2→2H++2e− (E0=0V) occurring on the catalyst of the anode and the chemical reaction of O2+4H++4e−→2H2O (E0=1.23V) occurring on the catalyst of the cathode is utilized to take out the current.
Therefore, unless the gas diffusion paths from the gas channels of the separators to the catalysts inside the catalyst layers over which the oxygen gas or hydrogen gas can move, the proton conduction paths over which protons (H+) generated on the anode catalyst can pass through the proton conductive electrolytic membrane to reach the catalyst of the cathode, and the electron conduction paths over which electrons (e−) generated on the anode catalyst can pass through the gas diffusion layers, separators, and external circuits to the cathode catalyst are continuously connected without being severed, it is not possible to efficiently take out the current.
Inside the catalyst layers, in general, it is important that the pores forming the gas diffusion paths formed in the interstices of the material, the electrolytic material forming the proton conduction paths, and the carbon material, metal material, or other conductive material forming the electron conduction paths respectively form connected networks.
Further, at the proton conduction paths in the proton conductive electrolytic membrane or catalyst layers, an exchange resin such as a perfluorosulfonic acid polymer is used as a polymer electrolytic material. The generally used polymer electrolytic material first exhibits a high proton conductivity in a wet environment, but ends up falling in proton conductivity in a dry environment. Therefore, to make a fuel cell operate efficiently, the polymer electrolytic material has to be kept in a sufficiently wet state at all times.
As one source of supply of the water for making the polymer electrolytic material a sufficiently wet state, there is the water generated by power generation at the cathode. However, the amount of generation of this water depends on the load conditions (current density). That is, at the time of stopping power generation or the time of low load operation, the amount of water which is generated is small, so the polymer electrolytic material dries out and the proton conductivity easily falls. On the other hand, at the time of high load operation, the amount of water which is generated is large, so the excessive water which the polymer electrolytic material cannot absorb easily blocks the pores forming the gas diffusion paths.
For a stable source of supply of water not dependent on the load conditions, a humidifier is generally used. The method of running the supplied gas through water warmed in advance to a certain temperature for humidification or the method of supplying water warmed to a certain temperature directly to the cell is used. However, to raise the energy efficiency of the system as a whole, it is preferable that no humidifier, which constantly consumes energy for holding the warmth, be provided. Even if there is one, consumption of the minimum necessary limit of energy is preferable. Further, to make the system as a whole lighter and small, no humidifier is preferable. Even if there is one, the minimum necessary limit of size is preferable.
Therefore, depending on the purpose of use of the fuel cell, sometimes it is not possible to mount a humidifier of a sufficient capacity on the system and not possible to sufficiently humidify the electrolytic material. Further, even when a humidifier provided with a sufficient humidification ability for steady state operation is mounted, the system will unavoidably temporarily fall into a low humidification state at the time of startup or at the time of load fluctuation.
In this way, the electrolytic material cannot necessarily be used in a suitable wet environment at all times, so there is a strong demand for a fuel cell catalyst layer which can exhibit a high performance under all sorts of load conditions or humidification conditions. A high performance fuel cell which is provided with such catalyst layers and therefore becomes easy to control and operate is also desired.
For the purpose of mainly avoiding drying out of the polymer electrolytic material, in the past the method has been proposed of using an ingredient having hydrophilicity for the gas diffusion layers or catalyst layers or the intermediate layers arranged between the gas diffusion layers and catalyst layers so as to maintain the wet state of the electrolytic membrane or the electrolytic material inside of the catalyst layers.
PLT 3 discloses, as a proposal for imparting hydrophilicity to the catalyst layers, to include zeolite, titania, or other hydrophilic particles or hydrophilic carrier in the anode so as to maintain a high cell performance even when lowering the amount of humidification.
PLT 4 discloses a fuel cell exhibiting superior startup characteristics even in a low temperature atmosphere wherein the catalyst layer of the anode contains a moisture retaining agent and wherein the moisture retaining agent is made a conductive material treated to make it hydrophilic (hydrophilic carbon black etc.)
PLT 5 discloses to provide a fuel cell-able to handle a broad range of humidification conditions by including, in the catalyst layers, hydrophilic particles carrying hydrophobic, particles such as silica particles carrying Teflon® particles.
PLT 6 proposes a fuel cell characterized by using activated carbon as the catalyst carrier, having a surface area SBET of the activated carbon according to the BET method (Brunauer Emmett Teller specific surface area method) satisfying SBET≧1500 m2/g, and having a ratio of the 2 nm or less size micropore surface area Smicro (m2/g) to the total pore area Stotal (m2/g) satisfying Smicro/Stotal≧0.5.
PLT 7 proposes a fuel cell using a carrier partially containing mesoporous carbon particles as the catalyst carrier.